Tracking marker support structure and surface registration methods employing the same for performing navigated surgical procedures

ABSTRACT

Devices and methods are provide for facilitating registration and calibration of surface imaging systems. Tracking marker support structures are described that include one or more fiducial reference markers, where the tracking marker support structures are configured to be removably and securely attached to a skeletal region of a patient. Methods are provided in which a tracking marker support structure is attached to a skeletal region in a pre-selected orientation, thereby establishing an intraoperative reference direction associated with the intraoperative position of the patient, which is employed for guiding the initial registration between intraoperatively acquired surface data and volumetric image data. In other example embodiments, the tracking marker support structure may be employed for assessing the validity of a calibration transformation between a tracking system and a surface imaging system. Example methods are also provided to detect whether or not a tracking marker support structure has moved from its initial position during a procedure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/054,784, titled “TRACKING MARKER SUPPORT STRUCTURE AND SURFACEREGISTRATION METHODS EMPLOYING THE SAME FOR PERFORMING NAVIGATEDSURGICAL PROCEDURES” and filed on Sep. 24, 2014, the entire contents ofwhich is incorporated herein by reference.

BACKGROUND

Surgical guidance enables surgeons to localize the position of surgicalinstruments relative to the human body without having complete visualaccess during surgery. Surgical guidance is routinely used in surgeriesthat involve anatomical locations such as the spine, brain, hip or otherorgans.

In general, surgical guidance consists of two steps: The first stepincludes the acquisition of a three dimensional (3D) data set of arelevant anatomical region of the body. This step may involve single ormultiple imaging modalities such as computed tomography (CT), magneticresonance tomography (MRT), positron emission tomography (PET) andultrasound (US). The 3D data set may be acquired before and/or duringthe surgical procedure. In the second step, the spatial position of thebody and the spatial relation of the surgical instruments to theposition of the anatomical region are tracked during the surgery. Thespatial position of this anatomical region is then mapped to its 3D dataset using specific image registration techniques. After registration,the spatial position of the surgical instruments as they are being usedby the surgeon can be displayed relative to the previously acquired 3Ddata set of the anatomical region. Surgical guidance systems usuallyincorporate the use of a reference structure which is affixed to thepatient in order to track patient motion and breathing so that tooltracking remains accurate during the procedure.

In some applications, optical-based systems are used for trackingspatial positions of tools and the reference frame during the surgery.These systems are based on two cameras that detect the positions of atleast three markers attached to the tracked surgical instruments andrequire line-of-sight from the cameras to the markers (for example,mounted with LEDs, or mounted with reflective probes). This necessitatesthe careful positioning of the cameras and design of tracked instrumentsso that line-of-sight is maintained during a surgical procedure.

SUMMARY

Devices and methods are provided for facilitating registration andcalibration of surface imaging systems. Tracking marker supportstructures are described that include one or more fiducial referencemarkers, where the tracking marker support structures are configured tobe removably and securely attached to a skeletal region of a patient.Methods are provided in which a tracking marker support structure isattached to a skeletal region in a pre-selected orientation, therebyestablishing an intraoperative reference direction associated with theintraoperative position of the patient, which is employed for guidingthe initial registration between intraoperatively acquired surface dataand volumetric image data. In other example embodiments, the trackingmarker support structure may be employed for assessing the validity of acalibration transformation between a tracking system and a surfaceimaging system. Example methods are also provided to detect whether ornot a tracking marker support structure has moved from its initialposition during a procedure.

Accordingly, in a first aspect, there is provided a method ofintraoperatively registering surface data with volumetric image data,the method comprising:

detecting, with a tracking system, signals associated with fiducialmarkers located on a tracking marker support structure, wherein thetracking marker support structure is removably attached to a skeletalfeature of a subject in a pre-selected orientation relative to theskeletal feature;

processing the signals and employing the pre-selected orientation todetermine an intraoperative reference direction associated with anintraoperative position and orientation of the subject;

intraoperatively acquiring the surface data from a surgical region ofinterest; and

employing the intraoperative reference direction when registering thesurface data to the volumetric image data.

In another aspect, there is provided a method of assessing the validityof a previously determined calibration transformation between a surfaceimaging system and a tracking system, the method comprising:

detecting, with the tracking system, signals associated with fiducialmarkers located on a tracking marker support structure, wherein thetracking marker support structure is removably attached to a patient,and acquiring surface data using the surface imaging system, wherein thesurface data is obtained from a spatial region that includes at least aportion of the tracking marker support structure;

processing the signals to determine a position and orientation of thetracking marker support structure;

determining, based on the intraoperative position and orientation of thetracking marker support structure, and based on the previouslydetermined calibration transformation between a reference frame of thesurface imaging system and a reference frame of the tracking system, aspatial subregion, in the reference frame of the surface imaging system,that is associated with the tracking marker support structure;

segmenting the surface data within the spatial subregion to obtain asegmented surface associated with the tracking marker support structure;

registering the segmented surface to reference surface datacharacterizing the surface of the tracking marker support structure,thereby obtaining a spatially registered reference surface; and

employing the spatially registered reference surface to assess thevalidity of the previously acquired calibration transformation.

In another aspect, there is provided a device for positioning fiducialmarkers relative to an exposed vertebrae, the device comprises:

a pair of forceps having a longitudinal axis associated therewith;

a pair of clamping jaws located near a distal region of the forceps,wherein the clamping jaws are configured to contact opposing sides of aspinous process when a force is applied to the forceps;

a locking mechanism operably connected to the forceps for removablymaintaining the forceps in a clamped configuration; and

a tracking frame having a proximal end connected to the forceps at alocation remote from clamping jaws, wherein the tracking frame supports,near a distal region thereof, the fiducial markers;

wherein the forceps extend from the clamping jaws such that when theclamping jaws are clamped to the spinous process, the longitudinal axisassociated with the forceps is angled relative to the Anterior-Posteriora normal direction that is associated with the subject, wherein thenormal direction lies in the sagittal plane and is perpendicular to theInferior-Superior direction of the spine, such that a skeletal regionadjacent to the skeletal feature is unobstructed by the forceps, therebypermitting overhead surface data acquisition of the skeletal region; and

wherein at least a portion of the tracking frame is angled relative thelongitudinal axis of the forceps, such that contact is avoided betweenthe fiducial markers and a user gripping the forceps.

In another aspect, there is provided a device for fixing fiducialmarkers relative to an exposed vertebrae, the device comprises:

a pair of forceps having a longitudinal axis;

a pair of clamping jaws located near a distal region of the forceps,wherein the clamping jaws are configured to contact opposing sides of aspinous process of the exposed vertebrae when a force is applied to theforceps;

a locking mechanism operably connected to the forceps for removablymaintaining the forceps in a clamped configuration; and

a tracking frame having a proximal end connected to the forceps at alocation remote from clamping jaws, wherein the tracking frame supports,near a distal region thereof, the fiducial markers;

wherein the clamping jaws are characterized by a normal axis that isperpendicular to the Inferior-Superior direction of the spine when theclamping jaws are clamped to the spinous process;

wherein the longitudinal axis of the forceps is angled relative to thenormal axis of the clamping jaws, and such that a skeletal regionadjacent to the skeletal feature is unobstructed by the forceps; and

wherein at least a portion of the tracking frame is angled relative thelongitudinal axis of the forceps, such that contact is avoided betweenthe fiducial markers and a user gripping the forceps.

In another aspect, there is provided a device for fixing fiducialmarkers relative to an exposed vertebrae, the device comprises:

a pair of forceps having a longitudinal axis;

a pair of clamping jaws located near a distal region of the forceps;

a tracking frame having a proximal end connected to the forceps at alocation remote from clamping jaws, wherein the tracking frame supports,near a distal region thereof, the fiducial markers;

a locking mechanism operably connected to the forceps for removablymaintaining the forceps in a clamped configuration;

wherein the clamping jaws are shaped to uniquely contact opposing sidesof a skeletal feature, such that the fiducial markers are oriented in apre-selected orientation relative to the skeletal feature.

In another aspect, there is provided a clamping device for clamping to aspinous process, the device comprises:

a pair of forceps having a longitudinal axis;

a pair of clamping jaws located near a distal region of the forceps;

a locking mechanism operably connected to the forceps for removablymaintaining the forceps in a clamped configuration;

wherein each clamping jaw comprises a clamping surface having twoco-planar outer flat surfaces and an inwardly directed surfaceconnecting the two outer flat surfaces, such that the clamping jaws areconfigured for clamping to a wide range of spinous process geometries,and wherein the outer flat surfaces and the inwardly directed surfaceeach comprise spikes.

In another aspect, there is provided a method of detecting a change inthe position and orientation of a tracking marker support structurerelative to a patient to which the tracking marker support structure isattached, the method comprising:

detecting, with a tracking system, signals associated with the fiducialmarkers located on the tracking marker support structure, and acquiringsurface data from a surgical region of interest using a surface imagingsystem;

determining the current position and orientation of the tracking markersupport structure based on the signals;

obtaining previously measured surface data from the surgical region ofinterest and an associated previously determined position andorientation of the tracking marker support structure;

registering the surface data with previously acquired surface data toobtain an intraoperative transformation;

comparing the intraoperative transformation to the shift between thecurrent position and orientation of the tracking marker supportstructure and the previously determined position and orientation of thetracking marker support structure and determining a change in theposition and orientation of the tracking marker support structurerelative to the patient.

In another aspect, there is provided a method of segmenting surface datato remove surface artifacts associated with an instrument havingfiducial markers attached thereto, the method comprising:

intraoperatively acquiring the surface data from a surgical region ofinterest using a surface imaging system;

detecting, with a tracking system, signals associated with the fiducialmarkers located on the instrument,

processing the signals to determine an intraoperative position andorientation of the instrument;

employing the intraoperative position and orientation of the instrument,and employing a calibration transformation between a reference frameassociated with the tracking system and a reference frame associatedwith the surface imaging system, to determine a suitable position andorientation of a cropping mask for removal of the surface artifactsassociated with the instrument; and

segmenting the surface data to remove the surface artifacts within theregion associated with the cropping mask.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will now be described, by way of example only, withreference to the drawings, in which:

FIG. 1A shows a schematic of an example surgical guidance system thatincludes an overhead integrated tracking system that employs structuredlight surface detection for image registration and optical tracking ofmedical instruments and medical devices with marker attachments.

FIG. 1B is a block diagram illustrating an example system configuration,including various example components of a control and processing unit.

FIG. 2 shows an example block diagram showing the components of atracking marker support structure.

FIGS. 3A and 3B provide an (A) isometric and (B) top view of an exampleembodiment of a tracking marker support structure.

FIGS. 4 and 5 provide side and top views, respectively, of the use of anexample tracking marker support structure for clamping the trackingmarker support structure in a pre-configured orientation.

FIGS. 6A and 6B illustrate an example embodiment of a tracking markersupport structure that employs a spring locking mechanism, where FIG. 6Bprovides a detailed view of the spring locking mechanism.

FIGS. 7A and 7B illustrate an example embodiment of a tracking markersupport structure that employs a thumb-screw mechanism, where FIG. 7Bprovides a detailed view of the thumb-screw mechanism.

FIGS. 8A and 8B illustrate an alternative example embodiment of atracking marker support structure that employs a thumb-screw mechanism,where FIG. 8B provides a detailed view of the thumb-screw mechanism.

FIGS. 9A-I show different example implementations of clamping jawsemployed by the gripping mechanism.

FIGS. 10A-H shows additional example implementations of clamping jawsbased on curved plates.

FIG. 11A is a flow chart illustrating an example method of employing atracking marker support structure to support the registration ofintraoperatively acquired surface data to volumetric (e.g.pre-operatively acquired) image data.

FIG. 11B is a flow chart illustrating an example method of employing atracking marker support structure to support the registration ofintraoperatively acquired surface data to volumetric image data formultiple vertebral levels.

FIG. 12A illustrates an example screenshot that can be employed forobtaining information regarding the intraoperative position of apatient.

FIG. 12B-I illustrate the use of different cropping masks which may beemployed for the segmentation of a surface within a spatial region orwithin a prescribed distance associated with the position of attachmentof the tracking marker support structure.

FIG. 13A illustrates an example method of employing a tracking markersupport structure for the intraoperative assessment of the validity of apreviously determined calibration transformation between a trackingsystem and a surface imaging system.

FIGS. 13B and 13C illustrate the use of cropping masks for thesegmentation of a surface associated with the tracking marker supportstructure when performing active calibration.

FIGS. 14A to 14E illustrate an active calibration process, in which atracking marker support structure is employed to verify the calibrationtransformation between the tracking system and the surface imagingsystem.

FIG. 15 illustrates an example implementation of a tracking markersupport structure that incorporates an additional surface withcharacteristic structures that provide additional non-symmetric surfacesuseful for the registration process.

FIG. 16 itemizes the characteristic features, and associated designconstraints, of an example tracking marker support structure.

FIG. 17 shows a generalized profile of an example tracking markersupport structure used for navigation of spinal procedures, identifyinga set of characteristic geometrical parameters.

FIG. 18 provides example values for the dimensions of the characteristicgeometrical parameters identified in FIG. 17.

FIGS. 19A to 19D illustrate an example implementation of a trackingmarker support structure based on the feature set shown in FIG. 16 andpertaining to cranial and/or maxillofacial surgical applications.

FIG. 19E shows a generalized profile of an example tracking markersupport structure used for navigation of cranial and/or maxillofacialsurgical procedures, identifying a set of characteristic geometricalparameters.

FIG. 19F provides example values for the dimensions of thecharacteristic geometrical parameters identified in FIG. 19E.

FIG. 20 shows a flow chart illustrating an example method in which asurface imaging based surgical guidance system is used to detect whetheror not the tracking marker support structure has been bumped or movedintraoperatively from its initial position.

FIG. 21 is a flow chart illustrating such an example method ofperforming selective surface segmentation based on known properties oftools or instruments that may be present within the field of view of asurface imaging system.

FIGS. 22A-22D illustrate an example implementation of a tracking markersupport structure based on the feature set shown in FIG. 16 andpertaining to cranial based surgical procedures.

FIG. 22E shows a generalized profile of an example tracking markersupport structure used for navigation of cranial based surgicalprocedures, identifying a set of characteristic geometrical parameters.

FIG. 22F provides example values for the dimensions of thecharacteristic geometrical parameters identified in FIG. 22D.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

As used herein, the terms “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions.Unless otherwise specified, the terms “about” and “approximately” meanplus or minus 25 percent or less.

It is to be understood that unless otherwise specified, any specifiedrange or group is as a shorthand way of referring to each and everymember of a range or group individually, as well as each and everypossible sub-range or sub-group encompassed therein and similarly withrespect to any sub-ranges or sub-groups therein. Unless otherwisespecified, the present disclosure relates to and explicitly incorporateseach and every specific member and combination of sub-ranges orsub-groups.

As used herein, the term “on the order of”, when used in conjunctionwith a quantity or parameter, refers to a range spanning approximatelyone tenth to ten times the stated quantity or parameter.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood to one ofordinary skill in the art. Unless otherwise indicated, such as throughcontext, as used herein, the following terms are intended to have thefollowing meanings:

As used herein, the term “position” refers to the location (e.g. x,y,z)of an object and its orientation (e.g. relative to one or morerotational axes) in three dimensions (3D) within a coordinate system.

As used herein, the term “tracking system” refers to a system thatallows the detection of the position of an object in three dimensions.An example of a tracking system is an optical tracking system operatingwith visual or infrared light that may employ stereo cameras to detectthe positions of passive optical markers (e.g. reflective spheres)and/or active optical markers (e.g. light emitting diodes (LEDs)). Othernon-limiting examples of tracking systems include electromagnetictracking systems and surface imaging tracking systems.

As used herein, the term “marker” refers to a locating indicator thatmay be affixed or otherwise connected to a flexible or rigid handheldimplement, patient, subject, instrument, tool, or other component of asurgical system or surgical field, and which is detectable by a trackingsystem for use in determining a position. A marker may be active orpassive, and may be detectable using an optical or electromagneticdetector. An example optical passive marker is a reflective sphere, orportion thereof, and an example active optical marker is an LED. Anotherexample of a marker is a glyph, which may contain sufficient spatialand/or geometrical co-planar features for determining athree-dimensional position and orientation. For example, a glyph markermay include at least three corner features, where the three cornerfeatures define a plane.

As used herein, the term “surface imaging system” refers to a systemthat detects the topology of a 3D surface (e.g. acquires a set ofsurface data describing the surface topology) within a field of view.Examples of surface imaging techniques include structured lightillumination, laser range finding, and photogrammetry.

As used herein, the term “calibration transformation” refers to atransformation that relates the coordinate system of a surface imagingsystem to that of a tracking system. The term “last calibrationtransformation” refers to the last valid or correct calibrationtransformation of the system. The last calibration can be determinedeither during the last service maintenance or by the system itself usinga validation step.

As used herein, the term “tracking marker support structure” refers to arigid structure including one or more fiducial or reference markers forintraoperative tracking, that configured to be securely attached to asubject (e.g. vertebra or the head), for example, to facilitate aregistration process.

FIG. 1A shows an illustration of an example of a surgical guidancesystem for tracking the intraoperative position of a medical instrumentrelative to patient anatomy during a spinal surgery. Patient 10 is shownin the prone (face down) position, with spine 15 exposed. Although thepresent example system employs a combination of an optical trackingsystem and a structured light surface imaging system, it will beunderstood that other types of tracking systems (i.e. non-optical) maybe employed, and that other types of surface imaging systems (i.e. otherthan employing structured light) may be employed.

The optical tracking subsystem is used to detect the position of medicalinstrument 40. In the example embodiment shown in FIG. 1, the opticaltracking subsystem includes stereo cameras with integrated infraredlighting 25 and attachment of highly reflective markers 65 to medicalinstrument 40. Due to their high reflectivity to infrared light, markers65 can be easily localized in each image of the two cameras 25. Theseimage positions are used to calculate the 3D position of each marker 65by geometrical triangulation. If at least three markers 65 are rigidlyattached to medical instrument 40, it is possible to compute itsposition (the six degrees of freedom—6-DOF). It is to be understood thatin some embodiments, less than three markers may be employed forposition tracking. For example, a single marker may be provided forposition tracking, provided that the single marker includes sufficientspatial structure and/or content. An example of such a single marker isa glyph including co-planar spatial features such as corner or edgefeatures.

In the example illustrations provided herein, markers 65 for the opticaltracking system are shown as reflective spheres, which are commonly usedfor passive optical tracking. However, any other type of markers, ormarker attributes, can be used depending on the used tracking systemsuch as, but not limited to LEDs, which do not require integration ofadditional lighting, reflective spheres, glyphs, varying marker color,varying marker size, varying marker shape.

The structured light imaging subsystem shown in the example embodimentis used to generate surface datasets. It includes at least oneillumination device 30 and at least one camera 35. The illuminationdevice(s) 30 project temporally and/or spatially modulated light ontothe surface to be imaged, while the camera(s) 35 capture images of theilluminated surface. This active illumination enables robust andefficient identification of pixel correspondences between calibratedcamera-projector (a projector may be thought of as an inverse camera) orcalibrated camera-camera system. The correspondence (disparity) data canthen be transformed into real-space coordinate data in the coordinatesystem of the calibrated camera(s) 35 and/or projector(s) 30 bygeometrical triangulation. During surgery, the structured light imagingsystem is positioned such that 3D surface of the surgical site (e.g. thebony surfaces of the exposed spine 15) is acquired. The created virtualrepresentation of the 3D surface is then registered to volumetric imagedata (e.g. CT, MRI, US, PET, etc.) by processing unit 50, using, forexample, methods described in International Patent Application No.PCT/CA2011/050257. The volumetric image data may be pre-operativelyacquired, but is not necessarily pre-operatively acquired. For example,in some applications, the volumetric image data may also beintra-operatively acquired.

FIG. 1B provides a block diagram illustrating an example implementationof a system for surface imaging. Volumetric data 95 is provided tocontrol and processing unit 50 for registration to intraoperativelyacquired surface data. Surface imaging system 92 scans object 1000, andsurface topology data is provided to control and processing unit 50,which is registered with volumetric image data 95. Tracking system 94 isemployed to track the positions and orientations of surgicalinstruments, and of a tracking marker support structure, as describedbelow. A calibration transformation is determined between the referenceframes of the surface imaging system 92 and the tracking system 94.

Surface imaging system 92 may be any suitable system for detecting,measuring, imaging, or otherwise determining the surface topology of oneor more objects using optical radiation or sound waves (e.g.ultrasound). Non-limiting examples of suitable optical devices includelaser range finders, photogrammetry systems, and structured lightimaging systems, which project surface topology detection light onto aregion of interest, and detect surface topology light that is scatteredor reflected from the region of interest. The detected optical signalscan be used to generate surface topology datasets consisting of pointclouds or meshes. Other examples using sound waves for determiningsurface topology can include ultrasonography.

FIG. 1B also provides an example implementation of control andprocessing unit 50, which includes one or more processors 70 (forexample, a CPU/microprocessoror a graphical processing unit, or acombination of a central processing unit or graphical processing unit),bus 72, memory 74, which may include random access memory (RAM) and/orread only memory (ROM), one or more internal storage devices 76 (e.g. ahard disk drive, compact disk drive or internal flash memory), a powersupply 84, one more communications interfaces 80, external storage 86, adisplay 78 and various input/output devices and/or interfaces 82 (e.g.,a receiver, a transmitter, a speaker, a display, an imaging sensor, suchas those used in a digital still camera or digital video camera, aclock, an output port, a user input device, such as a keyboard, akeypad, a mouse, a position tracked stylus, a position tracked probe, afoot switch, and/or a microphone for capturing speech commands).

Control and processing unit 50 may be programmed with programs,subroutines, applications or modules, which include executableinstructions, which when executed by the processor, causes the system toperform one or more methods described in the disclosure. Suchinstructions may be stored, for example, in memory 74 and/or internalstorage 76. In particular, in the example embodiment shown, registrationmodule 88 includes executable instructions for generating performingimage registration. For example, registration module 88 may includeexecutable instructions for performing the methods disclosed herein,such as the methods illustrated in FIGS. 11A, 11B, 13A, 20 and 21.

Although only one of each component is illustrated in FIG. 1B, anynumber of each component can be included in the control and processingunit 50. For example, a computer typically contains a number ofdifferent data storage media. Furthermore, although bus 72 is depictedas a single connection between all of the components, it will beappreciated that the bus 72 may represent one or more circuits, devicesor communication channels which link two or more of the components. Forexample, in personal computers, bus 72 often includes or is amotherboard. Control and processing unit 50 may include many more orless components than those shown.

In one embodiment, control and processing unit 50 may be, or include, ageneral purpose computer or any other hardware equivalents. Control andprocessing unit 50 may also be implemented as one or more physicaldevices that are coupled to processor 70 through one of morecommunications channels or interfaces. For example, control andprocessing unit 50 can be implemented using application specificintegrated circuits (ASICs). Alternatively, control and processing unit50 can be implemented as a combination of hardware and software, wherethe software is loaded into the processor from the memory or over anetwork connection. For example, connections between various componentsand/or modules in FIG. 1A, which enable communications of signals ordata between various systems, may be a direct connection such as a busor physical cable (e.g. for delivering an electrical or optical signal),such a LAN or WAN connections, or may be a wireless connection, forexample, as an optical transmission modality, or wireless transmissionmodality such as Wifi, NFC or Zigbee®.

While some embodiments have been described in the context of fullyfunctioning computers and computer systems, those skilled in the artwill appreciate that various embodiments are capable of beingdistributed as a program product in a variety of forms and are capableof being applied regardless of the particular type of machine orcomputer readable media used to actually effect the distribution.

A computer readable medium can be used to store software and data whichwhen executed by a data processing system causes the system to performvarious methods. The executable software and data can be stored invarious places including for example ROM, volatile RAM, non-volatilememory and/or cache. Portions of this software and/or data can be storedin any one of these storage devices. In general, a machine readablemedium includes any mechanism that provides (i.e., stores and/ortransmits) information in a form accessible by a machine (e.g., acomputer, network device, personal digital assistant, manufacturingtool, any device with a set of one or more processors, etc.).

Examples of computer-readable media include but are not limited torecordable and non-recordable type media such as volatile andnon-volatile memory devices, read only memory (ROM), random accessmemory (RAM), flash memory devices, floppy and other removable disks,magnetic disk storage media, optical storage media (e.g., compact discs(CDs), digital versatile disks (DVDs), etc.), among others. Theinstructions can be embodied in digital and analog communication linksfor electrical, optical, acoustical or other forms of propagatedsignals, such as carrier waves, infrared signals, digital signals, andthe like. As used herein, the phrases “computer readable material” and“computer readable storage medium” refers to all computer-readablemedia, except for a transitory propagating signal per se.

Some aspects of the present disclosure can be embodied, at least inpart, in software. That is, the techniques can be carried out in acomputer system or other data processing system in response to itsprocessor, such as a microprocessor, executing sequences of instructionscontained in a memory, such as ROM, volatile RAM, non-volatile memory,cache, magnetic and optical disks, or a remote storage device. Further,the instructions can be downloaded into a computing device over a datanetwork in a form of compiled and linked version. Alternatively, thelogic to perform the processes as discussed above could be implementedin additional computer and/or machine readable media, such as discretehardware components as large-scale integrated circuits (LSI's),application-specific integrated circuits (ASIC's), or firmware such aselectrically erasable programmable read-only memory (EEPROM's) andfield-programmable gate arrays (FPGAs).

In order to combine the tracking data with the surface data for surgicalnavigation, a calibration procedure is required, which relates thecoordinate system of the tracking system to that of the surface imagingsystem. If the relative position of the tracking system and the surfaceimaging system is fixed, this calibration may be performed by obtainingthe position of at-least 3 points from a calibration object from bothsystems, and aligning these points to obtain the calibrationtransformation, as described in International Patent Application No.PCT/CA2011/050257.

In an alternative embodiment, as disclosed in International PatentApplication No. PCT/CA2011/050257, the surface imaging device may havefiducial markers attached to it, which may be tracked by the trackingsystem. In this configuration, a calibration procedure can be used toobtain the calibration transformation from the coordinate system of thesurface system to the attached fiducial markers. The calibrationtransformation between the coordinate system of the tracking system andthe surface imaging system is then continuously updated as the positionof surface imaging device is changed.

After calibration, the calibration transformation between the coordinatesystem of the tracking system and the surface imaging system is known.Registering the surface datasets and volumetric image data is thereforeequivalent to identifying the position of the volumetric image data inthe coordinate system of the tracking system. As a result, any medicalinstrument 40, which is afterwards tracked with the tracking subsystem,can be presented to the surgeon as an overlay 55 of the surgicalinstrument 40 on the registered 3D image data on a display 60 or othervisualization devices.

A number of factors can affect the ongoing validity of the calibrationtransformation. For example, if the system were to undergo a significantmechanical impact, the relative positioning of the surface imagingsystem and the tracking system may shift slightly. In another example,the transformation may be dependent on the ambient temperature in whichit is operating and thus only valid within a specified range of ambienttemperatures. In both of these examples it would be advantageous tovalidate the accuracy of the calibration transformation and/or generatea new calibration transformation at the time of use without impactingthe surgical workflow.

While much of the discussion which follows assumes the use of a systemhaving two subsystems (tracking and surface imaging), it is noted thatalternative system configurations may be employed to performsimultaneous tool tracking and acquisition of anatomical surfaces usingan integrated system, for example by identification of surface topologyon tools, as described in International Patent Application No.PCT/CA2011/050257. In another example system configuration, a system canutilize a common pair of cameras for tool tracking (e.g. via glyphs orreflective spheres) and surface imaging (e.g. in either the visible orIR). Using the same camera systems for both tool tracking and surfaceimaging eliminates the need for the calibration between the two systemsdescribed above.

To compensate for patient or system motion, it is also advantageous touse a tracked device attached to the patient's anatomy (e.g. to askeletal feature of the patient's anatomy). Accordingly, as shown inFIG. 1, the position of a tracking marker support structure 45 isrecorded by the tracking system at the same time (i.e. within a timeduration that is sufficiently small to preclude errors associated withpatient motion) as when the surface dataset is acquired. The surfacedataset is transformed to the coordinate system of tracking system(using the previously acquired calibration transformation), and thenregistered to the volumetric image data. Subsequent tracking of surgicalinstruments relative to the volumetric image data can be performedrelative to the tracked tracking marker support structure, withcompensation for patient or system motion, without the need forcontinuous acquisition of surface data.

During a surgical procedure, it is generally preferred that trackingmarker support structure 45 should not block the line-of-sight on thesurgical target for the surgeon. The risk of possible obstructions ofthe surgeon's movement should be minimized especially when other trackedmedical instruments are in the surgical field, where the trackingattachments could shadow each other. It would also be beneficial for thesurgeon to be able to securely attach and to remove the tracking markersupport structure with relative ease. This is particularly important forspine surgery, where normally more than one vertebra are instrumentedand the risk of misplacing by accidentally touching the tracking markersupport structure by the surgeon is high. Furthermore, in order tominimize costs, a re-useable and sterilizable tracking marker supportstructure 45 is preferred. This can be achieved by use of appropriatematerials like for example stainless steel, tungsten carbide ortitanium.

For surgical guidance using a combination of a tracking system and asurface imaging system (as illustrated in the example system shown inFIG. 1), it will be understood that in order to acquire surface imagedata, tracking marker support structure 45 should not block theline-of-sight on the surgical target for the structured light system. Toachieve registration with the volumetric image data, the surface imagingsystem should cover the anatomical site of interest in a way that thecharacteristic anatomy is represented in the acquired surface data. Forexample in a navigated spine procedure, it will aid registration if theacquired surface captures the surfaces of the lamina and the spinousprocess in order to optimize the registration for a particular level ofthe spine (vertebrae). However, a tracking marker support structure canobstruct the visibility of the boney surfaces to the surface imagingsystem.

Attaching the tracking marker support structure to an adjacent vertebrallevel can avoid obstruction of the line-of-sight, but this can reducethe accuracy of the navigation, since the spine is flexible and therelative positions of the vertebras can change between the acquisitionof the preoperative images and when the patient is on the operatingtable. Therefore, it is beneficial to have a tracking marker supportstructure that can be securely attached to the vertebrae that is beingoperated on, while minimally obstructing the line-of-sight of thesurface imaging system to the relevant structures of that vertebrae.

FIG. 2 schematically illustrates an example tracking marker supportstructure 45 used for surgical guidance combing a tracking system and asurface imaging system for performing navigated spinal surgery. Exampletracking marker support structure 45 is shown including removablyattachable gripping mechanism 110, which firmly and removably attachesto the vertebrae of interest and avoids or reduces the obscuring of therelevant surfaces—the top of the spinous process and the laminas—fromthe line-of-sight of the surface system and the surgeon. Tracking markersupport structure 45 is also shown including locking mechanism 120,which ensures that the tracking marker support structure 45 remainssecurely attached to the vertebrae and can be readily attached andremoved. Tracking marker support structure 45 is also shown havingtracking frame 130 that includes fiducial/tracking markers, which aretracked by tracking system 94. As noted above, tracking frame 130 shouldnot interfere with the surgeon's use of tools in the vicinity of thevertebrae to which tracking marker support structure 45 is attached.

FIG. 3A shows an example implementation of a tracking marker supportstructure 200 which meets the above criteria for a combination oftracking and surface imaging. This tracking marker support structure 200is based on a bone clamp design. It employs forceps (which may bereferred to as a pair of forceps) comprising two members 205 that definelongitudinal axis 201 and pivot around a pin 210, such that jaws 215with spikes are rotated to grip the spinous process.

As shown in FIG. 3B, a locking mechanism is operably connected to theforceps. In the present example implementation, the locking mechanismincludes a series of interlocking teeth 220 cooperates with two handles225 on the other end of the members 205 to allow the surgeon to tightenand to lock tracking marker support structure 200 in place.

As shown in FIG. 3A, marker attachment 230 is provided that includestracking (fiducial) markers near a distal region thereof, where aproximal end of marker attachment 230 is mechanically coupled (e.g.attached, connected, or integrally formed) the forceps at a locationthat is remote from the location of clamping jaws 215, in order to allowthe tracking system to track the position of the tracking marker supportstructure. In the present example implementation, the tracking frame ismechanically coupled to one of the interlocking teeth 220, but it willbe understood that marker attachment 230 may be mechanically coupled toother portions of the forceps, such as to one of the handles, or to oneof longitudinal members 205. In this example embodiment, three passivereflective spheres are used as markers 240 for tracking the position ofthe tracking marker support structure. However, as noted above, it willbe understood that other configurations and types of fiducial markersmay be employed.

For clamping, the surgeon holds the tracking marker support structure200 with one hand 300 as indicated in FIG. 4. For example, the surgeonmay place the thumb 310 and the middle finger 320 through the twohandles 225. The index finger 330 may be employed to push against pivotpin 210, which helps to further stabilize the clamp 200 inside thesurgeon's hand 300.

As can be clearly seen in FIG. 3A and in FIG. 4, marker attachment 230is angled, relative to longitudinal axis 201, in a direction toward thepatient anatomy, thereby ensuring that the surgeon's hand 300 will notcontact marker attachment 230 or the markers 240 during clamping. Thisis a useful feature of the tracking marker support structure 200 becausethe surgeon's hand 300 might be covered by blood or other liquids, whichcould block the markers 240 and cause interference with the trackingsystem.

To attach or detach the tracking marker support structure 200 to thespinous process, the surgeon will adjust the clamping force of theinterlocking teeth 220 using the handles 225 and therefore the grip ofthe jaws 215 onto the interlocked bone. This locking mechanism can allowthe surgeon to change the position of the tracking marker supportstructure 200 between two spinous processes in a short duration, forexample, less than 10 seconds.

As shown in FIGS. 4 and 5, the members 205 of the forceps extend fromthe clamping jaws 215 such that when the clamping jaws are clamped tothe spinous process 410 of the vertebra of interest 440, thelongitudinal axis 201 associated with the forceps is angled relative tothe Anterior-Posterior (AP) direction 202 that is associated with thesubject, wherein the normal direction lies in the sagittal plane and isperpendicular to the Superior-Inferior (SI) 203 direction of the spine,such that a skeletal region 420 or 430 adjacent to the skeletal featureis unobstructed by the forceps, thereby permitting overhead surface dataacquisition of the skeletal region.

In the example embodiment shown in FIGS. 3A and 4, clamping jaws 215 arecharacterized by a normal axis 204 that is configured to beperpendicular to the SI direction 203 of the spine when the clampingjaws 215 are clamped to the spinous process 410. The jaws 215 aretherefore configured to uniquely clamp to the spinous process 410 in apre-selected orientation, such that the normal axis 204 of the clampingjaws 215 coincides with the AP direction 202. Accordingly, theattachment of tracking marker support structure 200 to the patientestablishes a reference direction that is associated with theintraoperative orientation of the patient. As described below, thisreference direction can be employed to guide the initial registrationprocess between a surface that is intraoperatively acquired by a surfaceimaging system and volumetric imaging data.

FIG. 5 shows the example tracking marker support structure 200 securelyattached to the spinous process 410 of a vertebrae 400. In thisconfiguration, tracking marker support structure 200 is not blocking theview of the surgeon onto the spinous process 410 and the left or theright lamina 420 and 430 respectively, and maintains a clear imagingfield for the surface imaging system. In addition, the marker attachment230 with the markers 240 is clearly visible to the tracking component ofthe combined navigation system.

It will be understood that the locking mechanism shown in FIG. 2 is butone example of a suitable locking mechanism, and that a wide variety ofalternative locking mechanisms may be employed. For example, FIG. 6A,illustrates an example embodiment of a tracking marker support structure500 that employs a spring locking mechanism 510. An extension spring 520pushes the two members 205 together, which tightens the jaws 215 on theopposite side of the pivot pin 210. The marker attachment 230 with themarkers 240 is connected to one of the members 205. As shown in a moredetailed view in FIG. 6B, extension spring 520 has a guidance wire orpin 530 that is received within extension spring 520. Guidance wire 530is connected to one spring stopper 540 on one of the members 205 andpasses through a hole or aperture in second spring stopper 550 on theother member 205. Another stopper 560 on the guidance wire 530 restrictsthe range possible movement of members 205. In order to attach thetracking marker support structure 500 to a spinous process, the surgeonopens the clamp by pushing apart the handles 225 and holds the referenceclose to the desired position on the spinous process. When the handles225 are released, the extension spring 520 is automatically clamping thetracking marker support structure 500 onto the spinous process.

FIG. 7A shows another example implementation of a tracking markersupport structure 600, which may be used for combined tracking andstructured light imaging. Again, two longitudinal members, 630 and 640respectively, with jaws 215 for clamping onto the bone are connected viaa pivot pin 210. In the present example embodiment, the markerattachment 230 with the markers 240 is a rigid extension of one of themembers 630 beyond the pivot pin 210. A thumb-screw mechanism 610 isused to tighten or loosen the grip of the clamp onto the bone and tolock jaws 215 in place. The mechanism is shown in more detail in FIG.7B. A threaded spindle 660 is positioned between two cylinder holders670, which are connected to the two members 630 and 640 of the clamp bya rotational axis 680. The surgeon can attach or detach the trackingmarker support structure 600 using the rotation wheel 690 on the spindle660. The thread on the spindle 660 is self-locking so that theattachment of the tracking marker support structure 600 is secure whenthe rotation wheel 690 is not used.

In an alternative example implementation, instead of positioning thethumb-screw mechanism 610 and the clamping jaws 215 on the same side ofthe pivot pin 210, they can be on opposite sides. For example, in theembodiment shown in FIG. 8A, the marker attachment 230 with the markers240 of tracking marker support structure 700 is connected directly tothe pivot pin 210 and integrates the rotation wheel 650 of thethumb-screw mechanism 710 using a slit 720 (for detailed view see FIG.8B).

The three example locking mechanisms described above (interlockingteeth, extension spring and thumb-screw) allow an easy, fast and secureattachment of the tracking marker support structure to the spinousprocess. However, as noted above, persons skilled in the art willunderstand that similar locking mechanisms may be employed.

FIGS. 9A-I show different jaw 215 designs for the gripping mechanism 110(see FIG. 2), which could be used, for example, to attach the trackingmarker support structure to a spinous process. FIG. 9A and show arectangular plate configuration for gripping flat surfaces such as thosefound in the lumbar and lower thoracic region of the spine. The surfaceis carrying a number of coned spikes, which increase the grip when thejaw is pressed onto the bone. The number and position of spikes may varyfor the specific design. The connection to the member 205 is on theshort side of the rectangular plate. However, it will be understood thatthis connection could be also be made on the long side of therectangular plate, for example, as shown in FIGS. 9C and D, if thetracking marker support structure should be employed for shorter spinousprocesses.

In other embodiments, the jaws may be configured to include two or morefingers. For example, FIGS. 9E and F show an example two fingerconfiguration which is more suitable for rough bone surfaces. FIGS. 9Gto I show an alternative example two finger configuration with foursloped spikes.

FIGS. 10A-H shows additional example jaw 215 designs based on curvedplates. An example angled bracket gripping plate, as shown in FIGS. 10Aand B are more suitable for gripping the rounded spinous processeslocated in the upper thoracic and cervical regions of the spine.

FIGS. 10C and D show a curved bracket also useful for the upper thoracicand cervical regions of the spine.

FIGS. 10E to H show a number of gripping plate configurations which arecapable of achieving good grip in any region of the spine due to thecombination of a flat plate region and curved or angled structures. Morespecifically, the example gripping plate (jaw) configurations shown inFIG. 10F and FIG. 10H each include co-planar flat surfaces 805 and 810,and also include an inwardly directed surface connecting the two outerflat surfaces 805 and 810, such that the clamping jaws are configuredfor clamping to a wide range of spinous process geometries. In FIG. 10E,the inwardly directed surface 820 is formed from two planar surfacesegments. FIG. 10H illustrates an alternative implementation in whichthe inwardly directed surface 830 is a curved surface. The outer flatsurfaces 805 and 810 and the inwardly directed surfaces 815 and 820 eachcomprise spikes.

It will be understood that the clamping jaw configurations shown inFIGS. 9A-I and FIGS. 10A-H may be provided with any type of surgicalclamping device, irrespective of whether or not the clamping deviceincludes a tracking frame. Furthermore, those skilled in the art willunderstand that a wide variety of alternative jaw (gripping plate)geometries and configurations may be employed in addition to the exampleimplementations shown in FIGS. 9A-I and FIGS. 10A-H.

In the example embodiments provided below, examples of the use of atracking marker support structure during surgical guidance aredescribed. It will be understood, however, that the use of the trackingmarker support structure, and the methods below, while being explainedwithin the example context of spinal surgical procedures, may be adaptedto, and employed in, a wide range of other surgical procedures. Examplesof additional surgical procedures that may benefit from the use of thepresent devices and methods disclosed herein are provided below.

In the present non-limiting example, at the beginning of a navigatedposterior approach spine surgery, the patient is placed in a prone(face-down) configuration on the operating table (see FIG. 1) andanesthesia is administered. The surgeon approaches the spine of thepatient from the back and exposes the boney surface of the vertebrae ofinterest by retracting soft tissue components.

Preparing the patient, the navigated portion of the surgery begins,which is illustrated in the example flow chart shown in FIG. 11A. Instep 1010, the tracking marker support structure 45 is securely attachedto the spinous process of the vertebrae to be navigated. In step 1020,the surface imaging system (such as a structured light system) acquiresa surface scan of the vertebrae, and the tracking system is employed torecord the position of the tracking marker support structure 45 usingtriangulation of the markers (e.g. passive optical fiducial markers 240shown in FIG. 3A).

In step 1030, surgical guidance system may be provided with registrationsupport information that may be to facilitate and/or improve theefficiency or accuracy of the registration of the acquired surface tothe volumetric (e.g. pre-operatively acquired) image data (as describedin further detail below). In step 1040, the registration processutilizes the acquired surfaces of the visible lamina and/or spinousprocess regions and the registration support information to register thevolumetric image data (e.g. from a CT scan).

Once the registration is complete, the system can present an overlaidimage, as shown in step 1050, of any tracked tool relative to theregistered volumetric image data for navigation of the surgicalprocedure on the vertebrae (e.g. insertion of pedicle screws). Thetracking marker support structure allows the surgical guidance system todetect, and compensate for, any movement (due to respiration, patientmovement, or system movement) of the vertebrae during the navigation,without requiring acquisition and registration of additional surfacedata to the volumetric image data. In step 1060, the surgeon removes thetracking marker support structure from the vertebrae and optionallyrestarts the process on the next vertebrae if desired. This process maythus be repeated one or more times to address one or more vertebrallevels.

FIG. 11B illustrates an example method for performing registration whenthe aforementioned process is repeated for an additional vertebrallevel. In this example method, the method of clamping and registering toa 1^(st) spinous process shown in FIG. 10A is repeated. However, afterremoval of tracking marker support structure 1060, the tracking markersupport is re-clamped to a 2^(nd) spinous process 1015. In step 1025 asecond surface scan and position is recorded by the surface imagingsystem and tracking system respectively. Position data from the trackingsystem acquired in step 1020 from the 1^(st) vertebral body is combinedwith position data acquired in step 1025 from the 2^(nd) vertebral bodyto calculate additional registration support data 1035. Examples of suchdata include estimates of axial direction of the spine (can be used asan initial condition for the registration 1040) and the approximatespacing between vertebral bodies (which can be used to specify croppingregion for the registration procedure 1040). It is noted that even ifthe tracking marker support is not moved to an adjacent level, it isstill possible to estimate the mean distance between vertebral bodiessince standard practice ensures the surgeon always specifies (through auser interface element) the level on which they have placed the clamp.

In one example implementation of the process illustrated in FIGS. 11Aand 11B, the surgeon or system operator may be queried to provide theregistration support information. For example, in step 1030, the surgeonor system operator may be requested to indicate a set of matched pointpairs on the pre-operative scan and the patient's body as initialinformation to guide the registration process (for example, three pointpairs may be requested and provided). The points can be selected, forexample, on the patient's body using a tracked tool touching thepatient's anatomy, or virtually on the acquired surface (touch-lessregistration). Typical point selection for spine surgery may include 1point on each of the left and right lamina and top of the spinousprocess (or the ligament which runs over it).

In another embodiment, a set of different registration supportinformation could be provided and employed in step 1030. For example,one piece of registration support information could be informationspecifying a particular anatomical direction in the acquired surface,for example the head-foot (superior-inferior) direction.

This information can be obtained by querying the surgeon or operator, orfor example, by inferring this direction through the positioning of thesystem relative to the patient. For example, if the system is positionednear the head of the operating table then the head-foot direction can beestimated with sufficient accuracy for registration. FIG. 12A shows anexample of graphical user interface, where the surgeon or systemoperator can specify the orientation of the system at the start of thesurgery. This information can be used together with the known patientpositioning during the pre-operative imaging (which is normally storedinside the data header) as registration support information (i.e. apriori information) to support the registration process.

In addition or alternatively, the surgeon or system operator can bequeried to enter the procedure specific information (e.g. surgery type,patient positioning, surgical approach or incision orientation) at thestart of the surgery using a graphical user interface similar to the oneshown in FIG. 12A. For example, a patient undergoing a posteriorapproach spine surgery will be in a prone (face down) position on theoperating table, which allows to infer the anterior-posterior directionin the acquired surface (since the system is always located above thepatient). This information could alternatively be obtained based on apre-determined surgical plan.

Another form of registration support information could be one matchedpoint pair selected on the pre-operative scan and the patient's body oracquired surface. A convenient point for a matched point pair could bethe top of the spinous process of the vertebrae of interest. Instead ofasking the surgeon or system operator to select the point on the spinousprocess, the known attachment point of the tracking marker supportstructure can be used. Assuming that the attachment point of the clampis always to the spinous process, the location of the spinous process onthe patient can be approximated using the tracked tracking markersupport structure position from the tracking system.

In several of the embodiments described herein, the tracking markersupport structure is configured to be attached a given skeletal featurein a known relative orientation. The skeletal feature may be a skeletalprojection, such as a spinous process. Such a skeletal feature has,associated therewith, a known anatomical direction in the sagittalplane. For example, in the example application of spinal surgicalprocedures, the tracking marker support structures described herein areconfigured to clamp to the spinous process such that the tracking markersupport structure is attached to the patient anatomy in a fixed positionand orientation relative to the point of attachment. For example, thetracking marker support structure shown in FIG. 5 is configured to clamponto the spinous process in a pre-selected orientation thatautomatically determines the inferior-superior direction of the spine.

This known orientation of the tracking marker support structure,relative to the patient anatomy, allows for the determination of anintraoperative reference direction associated with the intraoperativeposition and orientation of the patient. This intraoperative referencedirection may then be used, optionally with additional registrationsupport information (such as one or more matched point pairs), as aninput to the registration process, in order to improve the efficiencyand/or accuracy of the registration process. As noted above, as thevolumetric image data typically has orientation information in a headerfile, and therefore, determining an intraoperative reference directionassociated with the intraoperative patient orientation, and thus theintraoperative orientation of the acquired surface, can be beneficial inincreasing the efficiency and/or accuracy of the registration process.

For example, the intraoperative position and orientation of the patient(or at least of the local anatomical region of interest) can bedetermined based on the measured position of the tracking marker supportstructure, due to the known orientation of the tracking marker supportstructure relative to the skeletal feature, and the calibrationtransformation between the reference frame of the surface imaging deviceand the reference frame of the tracking system.

A full set of registration support information that is sufficient forthe registration process may require a combination of the abovementioned types of registration support information. As noted above, insome embodiments, the registration support information may includeinformation associated with the position and/or orientation of thetracking marker support structure, such as the position of attachment(that is associated with a known anatomical feature), and/or theorientation of the tracking marker support structure relative to theorientation of the known anatomical feature.

The surface imaging system has generally a field of view that is muchlarger than the exposed vertebrae of interest in order to enable thesurgeon to operate on multiple vertebrae levels without having toreposition the system each time. The additional surface regions outsidethe immediate vicinity of the vertebrae of interest generally do nothelp with the registration. Indeed, these additional surface regions canbe detrimental, potentially causing an incorrect registration, if thespine in the operating room is not in the same position as during thepre-operative imaging or if soft tissue surfaces at the surgicalincision borders are scanned.

In one example embodiment, the tracking marker support structure 45 isused to provide a spatial reference to determine where to segment theacquired surface, so that only the immediate surroundings of thevertebrae of interest is kept for registration.

Before this segmentation is performed, the spatial position of thetracking marker support structure 45 from the tracking system is firsttransformed into the coordinate system of the surface imaging systemusing the known calibration transformation between the two systems. Thesegmentation is then performed by cropping the surface data using asuitable mask within spatial region or within a prescribed distanceassociated with the position of attachment of the tracking markersupport structure. For example, a spherical mask surrounding the pointof attachment may be employed to determine the spatial region over wherethe acquired surface is to be cropped as per the segmentation process.

This segmentation creates a partial surface covering mainly thevertebrae of interest for the registration. Other masking geometries canbe used for the cropping of the surface data. Examples are rectangularboxes, cylindrical discs or other types of prisms with the main axisaligned to the spine, where the alignment can be determined from theposition of the clamping axis of the tracking marker support structure,which is aligned with the spinous process.

Examples of such cropping structures are shown in FIG. 12B-12G andinclude spherical, cigar shaped and patient specific cropping masks. Inthese examples it is useful to define the marker support structuretracking point 206 at the intersection of jaws 215 and members 210 onthe member connecting rigidly to marker attachment 230.

In FIGS. 12B and 12C, a spherical cropping region 208 is shown centeredon marker support structure tracking point 206. In FIGS. 12D and 12E, acigar shaped cropping mask 208 not centered on maker support structuretracking point 206 is shown. In FIGS. 12F and 12G, a patient specificcropping mask generated from CT scan data is shown. Lastly, in FIGS. 12Hand 12I a simple box cropping region centered on marker supportstructure tracking point 206 is shown.

These cropping masks may be used independently or in conjunction withone another at different stages of the registration process. Forexample, at an early stage of the registration process a large sphericalregion may be used to align multiple vertebral bodies in the surfacedata to volumetric images. In a second stage a cigar shaped croppingregion may be used to refine the registration of the specific vertebrallevel. Finally, in a third stage a tight patient specific cropping maskgenerated from the preoperative CT scan of the particular level (throughthe use of registration support information) can be used to furtherrefine the registration.

As mentioned before, the calibration of the surface imaging system totracking system enables surface imaging based surgical guidance.However, the validity of the calibration transformation can becompromised, if the relative position between the tracking system andsurfacing imaging system changed, for instance, due to physical impact.

In one example embodiment, the tracking marker support structure 45 isemployed to compute a real-time calibration transformation between thetracking system and the surface imaging system, for example, to assessthe validity of the previously determined calibration transformation. Asdescribed below, this can be achieved by performing surface detection todetermine the position and orientation of the tracking marker supportstructure in the reference frame of the surface imaging system, andcomparing this position with the position of the tracking marker supportstructure that is determined by the tracking system based on thedetection of signals from the markers, where the comparison employs thelast calibration transformation (the previously determined calibrationtransformation). The validity of the last calibration transformation cantherefore be assessed by determining whether or not the computedposition and orientation are within a prescribed tolerance.

This method may be performed at any time before or during a surgicalprocedure, such as at each time registration is performed, andoptionally each time a tracking marker support structure is attached toa new skeletal feature of a patient. For example, in the case of aspinal surgical procedure, the method may be performed or repeated whenthe tracking marker support structure (or an additional tracking markersupport structure) is attached to a new vertebral level.

This method will be referred to herein as “active calibration” and anexample process diagram is illustrated in FIG. 13A. The method includessome additional steps when compared to the process shown in FIG. 11after attachment 1010 of the tracking marker support structure to thespinous process and acquisition of a surface of the surgical field 1020.

For active calibration, as shown in FIG. 13A, the acquired surfaceshould include at least a portion of the tracking marker supportstructure 45, where the portion that is included has sufficient surfacetopology (i.e. includes one or more reference structures or surfacefeatures) to allow for the determination of the position and orientationof the tracking maker support structure via surface imaging. This isgenerally easily facilitated in the example case of a spinal surgicalprocedure because the tracking marker support structure 45 is typicallydirectly attached to the vertebrae of interest.

Assuming that the previously determined calibration transformation isstill sufficiently accurate, the transformation from the lastcalibration 1210 between the surface imaging system and tracking systemcan be used to identify a subregion within which to segment surface dataassociated with the tracking marker support structure from the acquiredsurface based on position tracked by the tracking system in step 1220.

Since the tracking marker support structure is normally an isolatedspatial structure, a simple cropping with a mask (e.g. a spherical mask)around the position predicted with the last calibration 1210 will likelybe sufficient in step 1220. However, other cropping masks can beenvisioned based on the known shape of the tracking marker supportstructure. FIGS. 13B and 13C depict examples of a spherical 1270 and amore conformal cropping mask 1280 for the marker support structure shownpreviously in FIGS. 3A and 3B.

Referring again to FIG. 13A, step 1230, the segmented tracking markersupport structure from the acquired surface is registered to referencesurface data characterizing the known surface of the tracking markersupport structure (for example, a 3D-model of the tracking markersupport structure or, for example, to a previously acquired surface ofthe tracking marker support structure) based on the position andorientation as currently measured by the tracking system, which yieldsin an active calibration transformation at the time of the surfaceacquisition 1020.

The active calibration is compared to the last calibration 1210 in step1240 and 1250. If the active and the last calibration transformation liewithin a specified tolerance, the last calibration transformation isdeemed valid and may be used for the following registration(alternatively, the new calibration transformation may be used forfuture imaging registration). However, if the calibrationtransformations do not agree within the specific tolerance, the lastcalibration transformation is deemed invalid. The last calibrationtransformation may be automatically replaced with the active calibrationtransformation in step 1260 (alternatively, a new calibrationtransformation may be performed using a calibration reference device).

After this decision, the registration process continues at step 1030, inwhich registration support information is received, and at 1040 in whichthe acquired surface is registered with the volumetric images (eitherusing the last calibration transformation—if valid—or with the updatedactive calibration transformation). The calibration transformation (lastor newly updated) may then be used, as shown at 1050, for the trackingof surgical tools. After the surgical procedure is complete, or when aportion of the surgical procedure is complete (e.g. the portionpertaining to the position of the anatomical feature to which thetracking marker support structure is fixed, such as a given vertebrallevel) and the tracking marker support structure may be removed from thespinous process as shown at 1060.

It will be understood that steps 1230 and 1240 of FIG. 13A may beperformed according to several different methods. For example, asdescribed above, a new calibration transformation can be calculated (theactive transformation), and compared to the last calibrationtransformation. In another example, the segmented surface data,registered to the reference surface data, can be used, with the lastcalibration transformation, to predict the current position of thetracking marker support structure, in the reference frame of thetracking system. This predicted position can be compared to the positionthat is currently measured by the tracking system. If the predicted andmeasured positions are within a prescribed tolerance, the lastcalibration transformation may be deemed to be valid. On the other hand,if the predicted and measured positions are outside of the prescribedtolerance, the last calibration transformation may be deemed to beinvalid, and a new calibration transformation may be computed thatresults in the predicted position agreeing with the measured position.It will be understood that the comparisons between the positions may bemade in the reference frame of the tracking system, or in the referenceframe of the surface imaging system, according to variations of theaforementioned methods.

FIGS. 14A-E show an example of the data outputs from the main steps ofthe process diagram illustrated in FIG. 13A. The surface image acquiredin step 1020 is shown in FIG. 14A. The tracking marker support structure1310 (showing some parts and the corresponding shadows) is attached to aspinous process 1320, which is going to be tracked after theregistration. The tracking frame 1330 with the markers 1340 is clearlyvisible in the surface image and is used in this example for the activecalibration.

Using the marker positions acquired by the tracking system and the lastcalibration transformation, a spatial subregion is identified that isassociated with the estimated position and orientation of the trackingmarker support structure, such that at least a portion of the trackingmarker support structure (in the present case, the tracking frame 1330)may be segmented in step 1220 from the surface image as shown in FIG.14B. The known 3D-model 1350 of the tracking frame is shown in FIG. 14C.If the last calibration is invalid or corrupted, the calibrationtransformation of the 3D-model 1350 to the surface image 1330 of thetracking frame based on the tracking data results in a clearmisalignment as shown in FIG. 14C. FIG. 14E shows the result after aregistration of the data shown in FIG. 14D in step 1230. This yields ina new calibration transformation which may be employed, after the steps1240 and 1250, as the active calibration transformation in step 1260.

In one example implementation of the aforementioned active calibrationmethod, the system may provide a warning to the surgeon or systemoperator in step 1260 (see FIG. 13), if in the calibration test 1240 and1250 the active and the last calibration transformation are notidentical within a specified tolerance. For example, the user might beasked to provide input instruction whether the registration should becontinued using the last calibration transformation, or using the activecalibration, or even aborted.

Although the active calibration method is described above using atracking marker support structure that is attached to an anatomicalstructure of the patient (e.g. a spinous process), it will be understoodthat in other example implementations, any other tool or tracking markersupport structure with known 3D-desing can be used for activecalibration, provided that the tool is tracked during the acquisition ofthe structural light and visible in the acquired surface.

In other example embodiments, the shape of the tracking frame (e.g.tracking frame 130 as shown in FIG. 2) can be designed so that thesurface imaging system will always acquire a reference surface that issuitable (or optimal) for registration. For example, reference surfacesthat may be incorporated into the shape of the tracking marker supportstructure include geometrical features such as pyramids, cubes, steps orchamfers, or other such features that ensure that the surface imagingsystem will acquire a surface from multiple possible views (i.e.relative positions between surface imaging system and tool).

For example, FIG. 15 illustrates an example implementation of a trackingmarker support structure 1400 that incorporates an additional surface1410 with characteristic structures 1420. These characteristicstructures provide additional non-symmetric surfaces useful for theregistration process. First, they enable the registration to be unique,whereas simple planar or spherical structures which have high degrees ofsymmetry may lead to registration ambiguity. Second, they reduce theprobability of overexposure by the surface imaging system and/or ambientlighting conditions on all characteristic structures simultaneously.Furthermore, surface properties (roughness/reflectivity) ofcharacteristic structures can also be tuned in order to optimize surfaceimage acquisition based on surface imaging system specification andambient environmental condition in which surface imaging system is meantto be used.

FIG. 16 itemizes characteristic features of the tracking marker supportstructure and provides a description as to how to select the parametervalues for a given surgical application.

FIG. 17 shows a generalized profile 1500 of a tracking marker supportstructure used for navigation of spinal procedures. The profile includesthe gripping jaws 215, the members 205, pin 210 connecting the members,and marker attachment 230. The arrow 1510 indicates the line-of-view ofthe combined tracking and a surface imaging system onto the trackingmarker support structure during an example intended use with a patientlying in the prone position.

FIG. 17 defines identifies a set of characteristic geometricalparameters of the example tracking marker support structure. Examplevalues for these dimensions for the example application of spinalsurgical procedures are specified in FIG. 18.

Referring to FIG. 18, the length 1520 and width 1525 of the grippingtips 215 are given by the typical dimensions of a spinous process, towhich the tracking marker support structure will be clamped. It can beadvantageous to maximize the overlap of the clamping surface of the jawswith the spinous process in order to counteract the torque and to ensurestable attachment of the clamp to the spinous process. It will thereforebe understood that a suitable size of the jaws 215 may depend on theanatomical regions of the spine (lumbar, thoracic and cervical) to whichthe device is to be attached. It will also be understand that thesuitable size of the jaws may vary depending on the patient subgroups(for example pediatric vs. geriatric vs. healthy adult).

Referring again to FIG. 17, in order to avoid blocking of theline-of-sight 1510 of the surface imaging system onto the laterallaminae, the thickness of the gripping jaws should be as small aspossible without compromising the mechanical integrity of the material.The angle 1530 subtended between normal direction 1540 and alongitudinal axis associated with members 210 should be greater thanapproximately 20° (e.g. between 20° and 40°), so that the trackingmarkers are not positioned directly above the surface of the spinousprocess.

It is also noted that pivot pin 210, which is located between members205, could potentially block the line-of-sight onto the spinous process.Therefore, a minimal distance 1570 between pivot point 210 and to jaws215 (along a longitudinal axis associated with members 205) can bebeneficial, depending on the angle 1530 of the members 205. On the otherhand, the necessary gripping force and mechanism as well as the spreadof distal arms when releasing the clamping mechanism will define theposition of the pivot pin 210.

As described above, the tracking marker support structure is intended totrack the motion of the patient, as characterized by motion of thespinous process. Therefore, tracking marker support structure should notcontact any other structures in the surgical cavity, which couldtransfer unwanted motion to the marker attachment 230. However, themarker attachment 230 requires a minimal profile size in order toachieve good tracking characteristics and might be close or even biggerthan the profile of the surgical cavity. It is therefore advantageousthat the marker attachment lie outside the surgical cavity when thetracking marker support structure is attached to the spinous process.

This can be achieved, for example, by positioning the marker attachment230 such that marker attachment 230 resides at a perpendicular offset1540 relative to of approximately 80 mm.

However, the overall size of the tracking marker support structureshould be as small as possible to avoid blocking the surgeon's movementor the placement of other surgical instruments, such as, for example, asurgical microscope. Therefore, the perpendicular offset 1540 of themarker attachment relative to the gripping tip 1530 should not be aboveapproximately 120 mm.

Another relevant issue is the potential for collision, shadowing orother interference between the tracking marker support structure andother tracked surgical instruments. Tracked surgical instrumentscommonly employ a set of fiducial markers that are positioned within aspatial region having a radius of approximately 40-70 mm relative to theshaft of the tracked instrument.

To avoid shadowing of such tracked tools by the marker attachment of thetracking marker support structure, the distance between markerattachment and jaws should be approximately 70 mm or more. This placesthe marker attachment at a distance that is sufficiently far from thesurgical region of interest to result in spatial interference withtracked surgical tools. This distance also ensures that the markerattachment 230 of the tracking marker support structure will not obscurethe line-of-sight for the surgeon or the structural light system 1510onto the vertebra.

Because of the potential for the marker attachment, which may includeaddition surfaces 1410 and addition characteristic structures 1420, toweigh significantly more than the rest of the tracking marker supportstructure, a longer distance between marker attachment and the grippingjaws increases the torque applied about gripping jaws, which coulddamage the clamped tracking marker support structure or require agripping force which might break the spinous process onto which it isbeing clamped.

As can be seen from FIG. 17, the horizontal distance D-1550 betweenmarker attachment 230 and gripping jaws 215 and the distance H-1540 ofthe marker attachment 230 relative to the gripping tip 215 directlydefine the direction of the members 205 and therefore the angle α 1530towards the gripping tip 215. The combined tracking and surface imagingsystem is normally positioned directly above the surgical cavity, whichallows a direct line-of-sight 1510 with minimal shadowing effects. Sincethe marker attachment 230 should be perpendicular to optical axis 1510to ensure optimal tracking, the angle of the marker attachment 1560should be in the range between 70° and 110°.

Although the angles shown in the examples provided herein are shown asfixed angles, it will be understood that any or all angles may bereplaced by adjustable angles having lockable joints which span theangular ranges specified or a subset of these ranges. Likewise, althoughthe lengths of various components and members shown in the examplesprovided herein are shown being fixed, it will be understood that any orall lengths may be replaced by adjustable lengths (e.g. via telescopicmembers that are slidably engaged) having two or more lockableconfigurations that span the length ranges specified or a subset ofthese ranges.

In will be understood that any or all angles, which are shown asdiscontinuities in the profile in FIG. 17, may be replaced by smootharcs or other shapes, which cover the same angular and distance range.For example, it will be understood that angles described and claimedherein may refer to the local angles at the point of attachment of onecomponent to another, or to virtual angles associated with theintersection of the longitudinal axes associated with variouscomponents.

Other tracking marker support structures designs based on the featureset described in FIG. 16 can be generated for different anatomicallocations. In fact, many of the realizations of tracking marker supportstructure shown in the examples provided herein can be employed inorthopedic shoulder surgery, where the tracking marker support structureis clamped to the spine of the scapula.

In other surgical applications, the tracking marker support structurecould be configured, for example, according to FIG. 16 and based on thelocal anatomy.

An example of a tracking marker support structure based on the featureset shown in FIG. 16 and pertaining to cranial and/or maxillofacialsurgical applications is shown in FIG. 19A and FIG. 19B. In this exampleimplementation, the tracking marker support structure 1600 comprises amouth guard like portion 1610 which is clamped inside the mouth toeither the upper (FIG. 19A) or lower (FIG. 19B) part of the jaw/teeth,depending on whether the lower member or the remainder of the skull isto be tracked. The tracking frame 1620 protrudes from the mouth suchthat it is visible to the navigation system and a screw based hingemechanism 1630 is used to lock the clamp in place. A detailed view ofthe back and the front of the mouth guard and the screw based hingemechanism is shown in FIG. 19C and FIG. 19D respectively. To connect thetracking marker support structure 1600, the mouth guard 1610 is pressedonto of the line of teeth. By tightening the two screws 1640 of thehinge mechanism 1630, the teeth is clamped between two fixation plates1650 and the inner rim of the mouth guard 1660. Loosening the screws1640, the marker support structure 1600 is removed from the teeth. Thisexample provides another illustrative embodiment of a tracking markersupport structure that is configured to attach to patient anatomy in apre-selected orientation, which, as described above, may be useful inproviding registration support information for use in performingregistration of acquired surface data with volumetric image data.

FIG. 19E shows a generalized profile of tracking marker supportstructure suitable for cranial and/or maxillofacial surgicalapplications. The profile's main features include marker attachment 1620and clamping jaws 1650. The arrow 1510 indicates the line-of-view of thecombined tracking and a surface imaging system onto the tracking markersupport structure during an example intended use with a patient held ina stereotactic frame in a supine position. Much of the same motivationfor features, dimensions and angles of generalized tracking markersupport structure 1500, which is suitable for spine surgery and shown inFIG. 17, also directly carry over to this application. Examples valuesfor dimensions and angles are shown in FIG. 19F. FIG. 22A-D shows anexample of a tracking marker support structure 2100 for neurosurgicalapplications. The tracking marker support structure 2100 is used afterthe soft tissue has been retracted from the skull and one or moreperforator holes have been made. The marker support structure hook 2110is inserted into one of the perforator holes 2125 with hook 2110positioned between the dura and the skull. During insertion the hook ispositioned pointing away from the skull flap 2140 such that clamping ismaintained after skull flap removal and good visualization of thecortical surface is maintained. The set screw 2105 is used to fix thetracking marker support structure into place using jaw 2120. Nextregistration to the skull surface is performed using the systems andmethods described above. Finally the skull flap 2140 is removed and thenavigated surgical procedure progresses in a standard fashion.Alternatively the surface of the brain or other internal structure couldalso be used for registration after skull flap 2140 has been removed.FIG. 22E shows a generalized profile of tracking marker supportstructure 2100 suitable for cranial procedures. Key features include jaw2325 and marker attachment 230. Examples values for dimensions andangles for the generalized profile shown in FIG. 22E are shown in FIG.22F. Dimensions for hook width is driven by the typical size of theperforator hole while the distance between the jaw and the hook isdriven by typical skull thickness. Other distances ranges are specifiedprimarily for not obstructing the line-of-sight of the tracking systemand the surgeon's range of motion.

FIG. 20 shows a flow chart illustrating an example method in which asurface imaging based surgical guidance system is used to detect whetheror not the tracking marker support structure has been bumped or movedintraoperatively from its initial position. The method involvesintraoperatively reacquiring the surface data for the anatomical regionof interest, and recording the current location of the tracking markersupport structure as shown in step 1710. The new surface data isregistered to the initially acquired surface data in step 1720, in orderto obtain an intraoperative transformation within the reference frame ofthe surface imaging system. The intraoperative transformation is thenemployed to estimate the current position and orientation of thetracking marker support structure in the reference frame of the trackingsystem, based on the previously known position of the tracking markersupport structure, as shown at step 1730. If there has been little or nomovement of the tracking marker support structure position relative tothe patient, then the transformations describing the tracking markersupport structure motion and patient surface motion between the two timepoints will lie within a pre-selected threshold. In other words, theintraoperative transformation can be compared to the difference betweenthe current and previous position and orientation of the tracking markersupport structure, in order to detect a change in the position andorientation of the tracking marker support structure relative to thepatient.

This check is performed in step 1740 with the output 1750 eithertriggering a warning (e.g. alerting a user of the system) andpotentially stopping tracking 1760, if the transformations aresignificantly different or allowing the tracking to continue if thechange in the relative position and orientation of the tracking markersupport structure lies within a pre-selected tolerance. This procedurecan be performed at any time after initial attachment of the trackingmarker support structure to the patient anatomy. For example, the methodmay be performed at a pre-selected frequency, or, for example, on demandas initiated by the surgeon or operator, or for example, each time a newstep in the surgical plan is to be executed.

It will be understood that the verification procedure described aboveand shown in FIG. 20 is equally valid if the surface imaging subsystemis used to measure the new position of the tracking marker supportstructure (for comparison with the estimated position). It is also to beunderstood that the active calibration procedure described in FIG. 13can also be applied in combination with the verification method in orderto simultaneously mitigate effects of relative motion between thesurface imaging subsystem and tracking subsystem.

In another embodiment, a method of data segmentation pertaining tosurface imaging surgical guidance system is presented. In someapplications, it may be advantageous to remove surface data pertainingto instruments tracked by the tracking system from the surface dataacquired by the surface imaging subsystem. This can be accomplished, forexample, by using a known shape or geometry (e.g. as provided byCAD/engineering design files or a known 3D model) of tools being trackedby the tracking subsystem.

In one example implementation, the method involves intraoperativelyacquiring the surface data using a surface imaging system, the surfacedata including surface artifacts associated with the surface of aninstrument, detecting, with a tracking system, signals associated withthe fiducial markers located on the instrument, and processing thesignals to determine an intraoperative position and orientation of theinstrument. The intraoperative position and orientation of theinstrument may then be used, along with the calibration transformationbetween the reference frames associated with the tracking system and thesurface imaging system, to determine a suitable position and orientationof a cropping mask for removal of the surface artifacts associated withthe instrument. The cropping mask, correctly positioned relative to thesurface data (e.g. where the cropping mask has been transformed into thereference frame of the surface imaging device), may then be employed tosegment the surface data to remove the surface artifacts within theregion associated with the cropping mask.

FIG. 21 is a flow chart illustrating such an example method ofperforming selective surface segmentation based on known properties oftools or instruments that may be present within the field of view of asurface imaging system. First, in step 1810, a tool-specific croppingregion is generated based on the known geometrical properties of thetool CAD file. As shown at step 1820, the tool position and orientationis determined (e.g. measured with the tracking system) when surface dataof the anatomical region is acquired. The cropping region is positionedand oriented based on the detected position and orientation of the tool,as determined based on data acquired from the tracking subsystem. Thiscropping region may be initially specified within the frame of referenceof the tracking subsystem, and then shifted into the coordinate systemof the surface imaging subsystem using the transformation linking thetwo subsystems, as shown at step 1830. Alternatively, the position andorientation of the tool within the reference frame of the surfaceimaging system, and the cropping region may be generated within thereference frame of the surface imaging system. The cropping region isthen used to reject points within the acquired surface data that liewithin the cropping region, as shown at step 1840. This method, orvariations thereof, may be employed to improve the quality androbustness of the registration process between surfaced data andvolumetric image data and/or surface data acquired at two or more timepoints (where surgical tools may be in two different locations).

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

1. A method of detecting a change in a position and orientation of atracking marker support structure relative to a patient, the methodcomprising: a) employing a surface imaging system to acquire firstintraoperative surface data associated with a surgical region ofinterest, the first intraoperative surface data being associated with afirst known position and orientation of the tracking marker supportstructure, wherein the tracking marker support structure is secured tothe patient; b) subsequently employing the surface imaging system toacquire second intraoperative surface data associated with the surgicalregion, the second intraoperative surface data being associated with asecond known position and orientation of the tracking marker supportstructure; c) registering the second intraoperative surface data withthe first intraoperative surface data to obtain an intraoperativetransformation; and d) processing the intraoperative transformation, thefirst known position and orientation of the tracking marker supportstructure, and the second known position and orientation of the trackingmarker support structure to determine the change in the position andorientation of the tracking marker support structure relative to thepatient.
 2. The method according to claim 1 further comprising alertinga user when the change in the position and orientation of the trackingmarker support structure relative to the patient exceeds a pre-selectedthreshold.
 3. The method according to claim 1 further comprisinginterrupting intraoperative guidance when the change in the position andorientation of the tracking marker support structure relative to thepatient exceeds a pre-selected threshold.
 4. The method according toclaim 1 further comprising performing steps b) through d) one or moretimes during a surgical procedure to monitor changes in the position andorientation of the tracking marker support structure relative to thepatient.
 5. The method according to claim 4 wherein steps b) through d)are repeated at a prescribed frequency.
 6. The method according to claim4 wherein steps b) through d) are repeated when performing a subsequentstep of a surgical plan.
 7. The method according to claim 1 wherein stepb) is initiated in response to input received from an operator.
 8. Themethod according to claim 1 wherein the tracking marker supportstructure comprises a plurality of fiducial markers, and wherein signalsassociated with the plurality of fiducial markers are employed todetermine the first known position and orientation of the trackingmarker support structure and the second known position and orientationof the tracking marker support structure.
 9. The method according toclaim 1 wherein the surface imaging system is employed to determine theknown second position and orientation of the tracking marker supportstructure by processing additional surface data associated with thetracking marker support structure.